Einstein’s life story is almost as well known as his science. He was born in 1879 into a middle-class Jewish family in southern Germany, and went to school in Munich, where he is supposed to have been an unsuccessful pupil and his Greek teacher predicted that he would ‘never amount to anything’. Albrecht Fölsing points out, however, that by 1929 the legend of Einstein’s poor academic record was so widespread that the principal felt obliged to dispel this bad publicity for the school by revealing his (quite respectable) examination marks in a letter to a Munich newspaper. Whatever the case, Einstein did not enjoy his school years, and when his parents moved away from Munich in 1894, escaped without graduating on the grounds that he was medically unfit for further schooling and that, where mathematics was concerned, he was up to graduation level already. After one more year at school in Switzerland he went to the Zurich Polytechnic to study for a diploma in mathematics – there, he was remembered by the teaching staff as a ‘lazy dog’. After receiving his diploma, he applied unsuccessfully for a number of junior academic jobs – within ten years many of the universities that had rejected him would be falling over themselves to hire him as a full professor – and it was only after a long spell of unemployment that he found a job in 1902 in the Bern Patent Office.

His spare time was spent working through the various problems in physics that had interested him as a student, with the aim of getting his name known through publications. The Patent Office, as he later acknowledged, played a key role in his development as a scientist: it forced him to think through problems with precision but also in broad terms, while enabling him to avoid the academic requirement to write superficial papers for journals. In the event, Einstein did produce an impressive number of papers, but they were far from superficial: the four he published in 1905 in Annalen der Physik must rate among the most important scientific publications of the century. Remarkably, he wrote these in near isolation, with few scientific colleagues to bounce his ideas off and only sporadic access to a library.

It’s impossible to talk about the 1905 papers without recourse to an uncomfortable number of superlatives. The first introduced the concept of light ‘quanta’, which challenged the accepted doctrine that light consists of waves of electromagnetic radiation. Ultimately, it was the most revolutionary of all Einstein’s papers, because it led to the development of quantum mechanics, and so to the overturning of Newton’s deterministic universe – a development with which Einstein never felt comfortable. The second paper explained the effect known as Brownian motion, where, for instance, under a microscope, smoke particles are observed to make erratic, jittery movements. Supported as it was by experimental work, Einstein’s theory provided final confirmation that air is composed of molecules, together with a method for determining the number of molecules in a given volume of air. While this may now sound rather quaint, before 1905 the existence of atoms and molecules was not taken for granted. This Brownian motion paper is one of the most frequently cited scientific publications of the 20th century, since the theory has many applications, ranging through physics, chemistry and biology: studies of the way aerosols disperse in the atmosphere are one instance.

The third and fourth papers of 1905 introduced the special theory of relativity. By assuming that the speed of light is constant throughout the universe, Einstein showed that the concepts of time and distance can never be absolute, because the motion of one moving body can only be measured relative to that of another. ‘It used to be thought,’ Einstein explained, ‘that if all things disappeared from the world, space and time would be left. According to relativity theory, however, space and time disappear along with the things.’ In other words, if two identical twins are separated, one staying on Earth, the other travelling the galaxy at speeds close to that of light, the space-bound twin will have aged only a few years on his return, while his brother will be an old man. This runs counter to our everyday understanding of time, but is a simple consequence of the fact that the speed of light has to be a constant for both twins, whatever their relative speeds.

This is as intriguing a result now as it was in 1905, but the principal achievement of relativity initially was to do away with the need for an ether, an invisible forcefield that was assumed to permeate the entire universe, providing a medium for light waves, rather as the surface of a pond provides a medium for ripples. An ether was a requirement of Maxwell’s theory of electromagnetism, itself one of the great successes of the 19th century, since it explained electromagnetic radiation (light, radio waves, x-rays etc) as a consequence of electric and magnetic forces unified into a single field. Crucially, the ether provided an absolute reference point for all moving objects, and thus was itself effectively at absolute rest. Einstein showed that the idea of absolute rest was meaningless and that Maxwell’s theory had no need of an ether. The concept was so entrenched, however, that many among the older generation of physicists had difficulty letting go of it.

Einstein’s ideas on light quanta met with opposition from physicists right across Europe, but the theories of relativity and of Brownian motion were accepted as major advances, despite the paradigm shift demanded by relativity. An academic career now became possible for Einstein, and in 1909 he was appointed professor of theoretical physics at Zurich University. In 1914, after several other moves, he was enticed to the Prussian Academy of Sciences in Berlin by a promise of not having to do any teaching. This was a major advantage, because by now Einstein was working obsessively on a general theory of relativity. Sometime in 1907, he had realised that all natural laws except those for gravity could be discussed within the framework of special relativity. Adapting the theory was not so simple, however, and it was not until 1916 that he finally succeeded. Despite the significant advance this represented, it was still too revolutionary for some of the old guard to take in, and when Einstein was awarded the Nobel Prize for Physics in 1921, it was specifically for his explanation of the photo-electric effect. In 1933, the rise of Nazism made his position in Germany untenable, and he left to join the newly formed Institute for Advanced Study in Princeton.

Apart from a brief diversion to develop what we now call Bose-Einstein statistics (which explain some unusual quantum effects that have to do with superconductivity and superfluidity), the general theory of relativity was Einstein’s last major contribution to physics. From 1916 until his death in 1955, he struggled fruitlessly with his Unified Field Theory, which would unite the then known forces of nature: gravitation and electromagnetism. These sorts of theory are now known as Grand Unified Theories (GUTs) or Theories of Everything. Einstein could never have succeeded because, we now realise, there are four forces needing to be unified (the strong and weak nuclear forces have joined the list). Even today, with an army of theoretical physicists and mathematicians working on the problem, it isn’t clear it can ever be solved.

Einstein was frequently called on to explain his work for the general public. Unfortunately, he was anything but a skilled expositor. In 1917 he wrote a popular book on relativity, using as little mathematics as possible. It was called On the Special and the General Theory of Relativity, Generally Comprehensible. Einstein himself was probably the first to joke that the title should have been ‘generally incomprehensible’. Einstein, Max Planck said, ‘believes that his books will become more readily intelligible if every now and again he drops in the words “Dear reader”.’ On his first visit to the United States, he travelled across the Atlantic with Chaim Weizmann, who reported: ‘During our crossing Einstein explained his theory to me every day, and on our arrival I realised that he really understands it.’

The beginning of Einstein’s rise to fame can be dated exactly: 6 November 1919, when the Royal Society announced the findings of two attempts to photograph the solar eclipse of 29 May 1919, and in doing so provided a striking verification of the general theory of relativity. The kernel of this is the principle of equivalence, which, put simply, means that gravity can be considered not only as a force, but also as a field, rather like Maxwell’s electromagnetic field. The advantage is that the motion of such bodies as planets or light waves in a gravitational field can be represented geometrically by adding time as a fourth dimension to the usual three spatial dimensions, thus producing the ‘four-dimensional space-time continuum’ beloved of Star Trek. The presence of mass – of a planet, say – produces a curvature of the continuum so that a moving body follows its contours.

To test the theory, Einstein made three predictions, all accessible to experiment. The first concerned anomalies in the motion of the planet Mercury around the Sun, which had been noticed in the 19th century and couldn’t be explained by Newtonian physics. General relativity accounted for these anomalies straight away. The second prediction, that, rather than being straight, the path of light is deflected, or curved, in a strong gravitational field such as that around the Sun, was confirmed by the photographs of the eclipse. The third prediction was that light from a star moving away from the Earth would undergo a change in wave-length and appear ‘redshifted’; this was not confirmed until five years after Einstein’s death. The eclipse observations had necessitated measuring the angle of deflection of light from a star passing very close to the Sun, by making a comparison of its position in the sky before and during the eclipse. The results were in close agreement with the angle as calculated from general relativity. Einstein was immediately catapulted to international (and unwelcome) fame.

In A Brief History of Time, Stephen Hawking claims that later examination of the eclipse results has revealed that the agreement between experiment and theory had been entirely fortuitous, since the margin of experimental error was actually as large as Einstein’s predicted angle of deflection. I’ve been unable to trace any evidence to support Hawking’s assertion, since even in fairly recent reviews of light deflection experiments, the error for the 1919 experiment is given as representing no more than 30 per cent of the observed value. But if Hawking is right, the eclipse experiment was a prime case of scientists making sure they got the results they wanted – a more common occurrence than one might hope. Interestingly, Hawking helped to whip up a similar frenzy over the first results obtained from the COBE (Cosmic Background Explorer) satellite. Modern ideas of the evolution of the universe require that the microwave background radiation (the radiation flung into space by the Big Bang) is spatially inhomogeneous, to account for the fact that matter in the universe is not evenly dispersed but condensed into stars and planets. The COBE satellite was designed to measure this inhomogeneity by making a temperature map of space. The first results, reported in 1992, showed ripples in the background radiation, as expected. The resultant media hype was extreme, prompted partly by Hawking hailing the COBE results as ‘the scientific discovery of the century, if not all time’. Perhaps he had forgotten about Einstein’s 1905 papers. It turns out, however, that the ripples were largely artefacts of the experimental errors, just as Hawking claimed was the case with the 1919 data. Only by taking into account all of the COBE data could it be shown that there was a slight statistical tendency for an inhomogeneous (rather than homogeneous) background. Much more accurate tests of general relativity have in fact been made since 1919, all of them confirming the theory, which was never in any doubt in Einstein’s mind. Once, when a student asked what he would do if it turned out that general relativity was wrong, he said: ‘In that case I’d have to feel sorry for God, because the theory is correct.’

Given that Einstein was frequently praised as the greatest genius since Newton, and compared favourably with Kepler, Copernicus, Galileo etc, it is surprising to find that many of his important theoretical developments were anticipated in whole or in part by others. The mathematics of special relativity was developed almost in its entirety by Poincaré and Lorentz, for instance, independently and in advance of Einstein. His real contribution to science was not the mathematical so much as the conceptual advances he made. He dared to redefine basic concepts such as time and space and in so doing solved problems concerning light and electromagnetism that had been around for centuries and become so familiar they were barely even acknowledged as problems. Yet there have been others this century who did as much to redefine physics, notably Dirac, Bohr and Heisenberg, who formulated quantum mechanics. So why does Einstein receive all the attention?

In his Einstein Lived Here Abraham Pais explored his extraordinarily wide appeal and reached the conclusion that it comes largely from the media’s fascination with relativity: ‘The drama of Einstein’s emergence was enhanced by the exhaustion and chaos following the conclusion, just one year earlier, of the First World War, which had caused millions to die, empires to fall, leaving mankind in a state of uncertainty. Just in those days a new figure appears abruptly, carrying the message of a new order in the universe.’ Coupled to this was the ‘mystery of non-understanding’ of relativity theory, which even Einstein recognised as a principal factor in his popularity.

Quantum mechanics offers even more of this kind of mystery. It is at once appealing and repellent, and, as its creator, Einstein felt the ambivalence more than most. Its basic principles can be used to explain not only the structure and properties of atoms, including the way they interact in molecules, solids and liquids, but also those of the nuclei and such subatomic particles as the electron and proton. There is currently nothing to suggest that the fundamental ideas of quantum mechanics are wrong. Indeed, one development of it, quantum electrodynamics, is the most precise physical theory ever developed, agreeing with experimental results to 1 part in 100,000,000. But its conceptual basis is bizarre, and it has forced a complete reappraisal of the way we understand the universe. Classical physics was based on the ideas of causality and determinism, but Heisenberg’s Uncertainty Principle, which states that there is a limit beyond which we cannot see, no matter how sensitive and precise the experiment, is a fundamental law of quantum mechanics. As a result, familiar concepts like size, momentum, time and energy become imprecise or even meaningless. This is epitomised in the Copenhagen interpretation of quantum mechanics, which emphasises the ineluctable participation of the observer in determining the outcome of a measurement. Einstein reacted to the Uncertainty Principle with scorn: ‘Heisenberg has laid a big quantum egg.’

As the weirdness inherent in quantum theory became more apparent, Einstein rejected the idea that it is complete, and instead immersed himself in the search for a unified field theory, hoping to provide an exact solution where quantum mechanics was only an approximation. He became obsessed with ever more abstract mathematics, believing that the only reliable criteria for a workable theory were mathematical simplicity and elegance rather than a basis of empirical fact. At first, many of his colleagues were supportive, but as time went on his isolation grew. At Princeton, the younger scientists were warned that ‘it would be better not to work with Einstein,’ and Robert Oppenheimer reported to his family that ‘Einstein is completely cuckoo.’

One constructive consequence of Einstein’s opposition to quantum mechanics was that it forced Niels Bohr to formulate his ideas more precisely. In 1935, Einstein, Boris Podolsky and Nathan Rosen wrote a paper setting out a paradoxical thought experiment intended to demonstrate that quantum mechanics gives only an incomplete description of physical reality. The argument involves two electrons that have collided and flown apart to such a great distance that no interaction can occur over a reasonable timescale by light travelling between them. The law of conservation of momentum means that, if you measure the momentum of one electron, that of the other can be calculated without the act of measurement disturbing it. The position of the first electron is then determined by a second measurement, this time specifying the position of the second electron without disturbing it. Thus, both the momentum and position of the second electron can be determined, making them part of ‘physical reality’, which contradicts the Uncertainty Principle and the basic tenets of quantum mechanics.

Bohr’s response made it clear that it is impossible to speak of ‘physical reality’ without including the measuring apparatus as itself part of this reality, thus vindicating the laws of quantum mechanics. The original thought experiment has now been performed several times, and Bohr’s arguments shown to hold true in a spectacular fashion. The most recent experiment was done in Geneva in 1996, when pairs of photons (Einstein’s light quanta) were sent along separate optical fibres towards detectors ten kilometres apart. Measurements on each pair were made almost simultaneously, so that no signal, not even one going at the speed of light, could have travelled the ten kilometres between the photons. The photons were found to be ‘entangled’: that is, a measurement on one instantaneously affected the state of the other.

Einstein was not alone in attacking quantum mechanics. Erwin Schrödinger had similar reservations, and suggested the paradox known as ‘Schrödinger’s cat’. In this thought experiment, a cat is placed in a sealed box together with a lethal device triggered by the decay of a radioactive atom, which is quantum mechanical and so occurs over a random time interval. In order to determine whether the atom has decayed, we would have to open the box and see whether the cat is alive or dead. But according to the standard Copenhagen interpretation, before we actually make an observation of the cat, the state of the whole system (atom, lethal device and cat) is indeterminate. In other words, while the cat is hidden in the box, it is neither alive nor dead. If we reject this obviously absurd conclusion, we must ask at what point quantum mechanics fails and classical physics takes over. This was something even Bohr couldn’t answer, but as with the entanglement paradox, recent work seems to be finding a resolution to Schrödinger’s cat, within the remit of Bohr’s understanding of quantum mechanics.

Denis Brian is intent on portraying Einstein as the archetypal absent-minded professor, and gives a lot of space to his political and pacifist pronouncements, while dealing brutally with his major discoveries. Fölsing is more balanced, recognising that the science is the real hero of the story, but the virtue of his book is that it presents Einstein in the context of the early 20th century, and shows him to have been the founding father of what we now call the New Physics.